Boresighting optical systems (“boresights”) are used for long-range weapons, such as rifles, to allow the weapon's user to view a target and align the weapon relative to the target, e.g., to a select bullet impact point. A typical boresight includes an alignment mechanism used to align the weapon and boresight relative to the target, e.g., to the desired bullet impact point. An example alignment mechanism is a cross-hair reticle wherein the boresight is adjusted (“aligned”) so that the reticle cross-hairs match the desired bullet impact point for a given target distance. Additional adjustments may be made to realign the boresight to the bullet impact point to take into account such factors as windage, distance, and bullet caliber. Finding the proper boresight alignment for a given set of conditions often involves trial and error, which is time consuming and not always convenient. Consequently, one would generally prefer to perform boresight alignment and/or realignment as infrequently as possible.
Certain types of boresights have both day-vision and night-vision capability, and are referred to herein as “night-day boresights.” Night vision capability is provided by a night-vision optical system, referred to hereinafter as “night optics.” Likewise, the day-vision capability is provided by a day-vision optical system, referred to hereinafter as “day optics.” Because night optics have different imaging capabilities than day optics and includes an image intensifier, the optical design of the night optics is different than the day optics. Accordingly, the night optics and the day optics are typically separate optical systems, even when combined in the same housing.
There are three basic approaches to using both night optics and day  optics on the same weapon. The first approach involves having separate night optics and day optics boresights and simply replacing one with the other as desired. However, this requires “rezeroing” each boresight every time it is installed on the weapon. Also, it is not always convenient to swap boresights, such as in combat or hunting situations since, among other things, there is often precious little time to rezero and align (e.g., by shooting at a target) after replacing one boresight with another.
The second approach involves integrating the day and night optics by combining the night optics and day optics optical paths using an adjustable mirror or beamsplitter. While this approach does not require physically swapping the night and day optics, it is still problematic because it requires boresight realignment when switching from the night optics to the day optics.
The third approach involves “clipping on” the night optics to the day optics. In one version of this approach, the night optics is clipped to a mount that holds the night optics above the day optics. A beamsplitter or mirror is then mounted in front of the day optics to project the night image down from the night optics into the day optics. The position of the mirror is then adjusted to obtain the required boresight alignment. Typically, the mirror or beamsplitter is adjusted until the two lines of sight are parallel, being offset by the difference in mounting height. A predictable point of impact is then available when using the night optics to augment the day optics for night time use.
In another version of the third approach, the clip-on night optics is mounted in front of (or “in line with”) the day optics. In this arrangement, the night optics is said to be optically “upstream” of the day optics, i.e., the night optics is closer to the target and so receives light prior to the day optics.
FIG. 1 is a schematic diagram of an in-line night-day boresight 10 shown mounted to a platform, such as a weapon barrel 14. Boresight 10 includes removable night optics 20 having an input end 22 and an output end 24. Night optics 20 is arranged upstream of and in line with day optics 30 for nighttime viewing and is removed for daytime viewing. Day optics 30 has an input end 32 adjacent the night optics output end 24, and an output end 34 opposite input end 32. Night optics 20 and day optics 30 are arranged along an optical axis A1. A user 50 is shown viewing through the boresight at output end 34 of day optics 30.
In the operation of boresight 10, light 52 from a distant target (not shown) enters the input end 22 of night optics 20 and is incident an image intensifier tube 56, which outputs intensified (amplified) light 60. The intensified light 60 is then relayed to input end 32 of day optics 30 and is relayed to output end 34 to be viewed by user 50. While night optics 20 is designed for use in combination with day optics 30, the inevitable manufacturing errors (e.g., mechanical misalignments and tolerance errors) in night optics 20 cause light 60 to take a different path 61 (dashed line). The difference in paths 60 and 61 corresponds to an image shift IS of an amount Δ as seen by user 50. This image shift typically corresponds to an angular error of about 10 to 15 minutes (i.e., ˜10′-15′) of arc.
Reducing or eliminating this image shift has been achieved in several different ways. One way is to determine the alignment error due to the image shift due arising from the presence of the night optics, and dial this error into the day optics. Unfortunately, this approach is not preferred because the user has a different set of adjustments when using the night optics and the day optics. Another way is to adjust the optical centerlines during assembly to keep the image shift within a usable margin of error. While this can work in principle, it adds cost to the assembly and testing, with the latter having to be performed frequently until the assembly “settles” due to weapon shock. Yet another way is to provide mechanical adjustment capability to the night optic and day optic mounts. While this is a straightforward solution to reducing or eliminating the resultant image shift, it is not desirable because realignment needs to be performed every time the night optics is mounted and dismounted.